Metal fabrication



May 7, 1963 A. F. JOHNSON METAL FABRICATION 3 Sheets-Sheet 1 OriginalFiled April 7, 1955 INVENTOR. ARTHUR F. JOHNSON ATTORNEY May 7, 1963 A.F. JOHNSON METAL FABRICATION 3 Sheets-Sheet 2 Original Filed April 7,1955 INVENTOR. ARTHUR F. JOHNSON BY ATTOR NEY May 7, 1963 A. F. JOHNSON3,088,193

METAL FABRICATION Original Filed April 7, 1955 5 Sheets-Sheet :5

Fig. II

INVENTOR. ARTHUR F. JOHNSON ATTOR N EY atent 3,088,193 Patented May 7,1963 3,088,193 METAL FABRICATION Arthur F. Johnson, Boulder, Colo.,assignor to Olin Mathieson Chemical Corporation, East Alton, 111., acorporation of Virginia Griginal application Apr. 7, 1955, Ser. No.499,956, now Patent No. 2,915,296, dated Dec. 1, 1959. Divided and thisapplication Apr. 13, 1959, Ser. No. 809,334

13 Claims. (Cl. 29-1573) This invention relates generally to metalfabrications and more particularly to a novel heat exchanger. Thisapplication is a division of pending application Serial No. 499,956,filed April 7, 1955, now Patent No. 2,915,296.

In manufacturing liquid to air type heat exchangers it is desirable touse the least amount of metal possible to achieve the area of heatexchange surface required to transfer the heat from the liquid to theair. Heat exchangers of this type ordinarily have a tube containing theliquid to be cooled and a plurality of fins extending radiallytherefrom. The transfer of a given amount of heat from metal to airrequires a much larger surface area than is required to transfer thesame amount of heat from the liquid to the metal. Consequently, the finsordinarily make up the larger part of the heat exchanger. The ultimatecost of the heat exchanger is thus largely dependent upon the amount ofmetal required to provide suflicient fins to achieve the proper heattransfer from the fins to the air.

Therefore, an object of this invention is to provide a heat exchangerwith a novel fin having improved heat transfer properties. Anotherobject of this invention is to provide heat exchange units having finsconstructed from metals or alloys which have inferior specific heatconductivity but are economically advantageous. Another object of thisinvention is to provide a fin for heat exchangers having an improvedmeans for conducting the heat from a liquid to air. Still another objectof this invention is to provide a liquid to air heat exchanger havingfins adapted to more efficiently transfer heat from a confined fluidinto the surrounding air. A still further object of this invention is toprovide a method for making liquid to air heat exchangers.

Other objects will become apparent from the following detail descriptionand the attached drawing in which FIGURE 1 illustrates in a perspectiveview an assembly of metal sheets suitable for forming a fin for heatexchangers in accordance with an embodiment of this invention;

FIGURE 2 is a plan view of the assembly of FIGURE 1 after welding;

FIGURE 3 is a cross-sectional View taken along the line A--A of FIGURE2;

FIGURE 4 illustrates a fragmentary elevational view the embodiment ofFIGURE 2 after inflation;

FIGURE 5 is a fragmentary perspective view partially in section alongthe line B-B of FIGURE 4 illustrating the step of filling the hollowchannel of the assembly;

FIGURE 6 illustrates an embodiment of this invention such as can beformed from the panel of FIGURE 4;

FIGURE 7 illustrates in a perspective view an assembly of two metalsheets prepared for welding to form another embodiment of thisinvention;

FIGURE 8 illustrates an embodiment of this invention formed from anassembly like that of FIGURE 7;

FIGURE 9 is a cross-sectional view illustrating the embodiment of FIGURE8;

FIGURE 10 is a cross-sectional view of an embodiment of this inventionformed from three sheets of metal; and

FIGURE 11 illustrates still another embodiment of the invention.

In accordance with this invention, the foregoing objects as well asothers are accomplished, generally speaking, by providing a heatexchanger fin having a tubular circuit containing a fluid refrigerantextending from the heated portion of the fin to the portion in thecooling medium. The circuit or conduit is of such configuration that thefluid contained therein can circulate from a point adjacent the heatedliquid to the outer edge of the fin and back either entirely in theliquid phase or in both liquid and vapor phases. The conduit may extendaround the edges of the fin or it may be of such configuration that oneportion thereof extends along the hotter edge of the fin and anotherportion extends along the colder edge with radially extending portionsconnecting the two together. The outer edge of the fin is at a greaterelevation than the edge adjacent the tube containing the heated liquidin order that the confined fluid after cooling by the air or othermedium in contact with the tin can return by gravity to that portion ofthe conduit adjacent the heated liquid. Any suitable fluid maybe usedbut for most purposes a fluid having a boiling point less than that ofthe liquid to be cooled is preferred so that cooling may be accomplishedat least partly by phase transitions in the fluid between the liquid andvapor phases.

In practice, the high boiling point liquid which is to be cooled isflowed through a tube or other means having attached thereto the fins ofthis invention. Ordinarily, a plurality of the fins are utilized and theupper surface of each of the fins forms an acute angle with the tube.The conduit of each fin is only partially filled with the fluid. Asstated above, it is preferred that the refrigerant have a boiling pointless than that of the liquid to be cooled in order that it will beheated above its boiling point. This sets up a circulation in theconduit and the heated and less dense fluid flows towards the outer edgeof the fin Where it is cooled. The more dense, cooler fluid flows bygravity back towards the inner edge of the fin adjacent the heatedliquid where it again absorbs heat and the process is repeated. Sincethe fluid expands with increase in temperature, and particularly at itsboiling point, it is advantageous in some embodiments to make theportions which will contain the heated fluid of greater volume thanthose portions containing the fluid after it has been cooled. A conduitof gradual tapering width accomplishes this purpose by locating thewider portion near the heated tube and the narrower portion along theopposite edge of the fin. If only one tubular portion extends radiallyacross the fin the fluid may be refluxed with the cooler fluid flowingback into the boiler portion of the conduit for reheating. When coolingby transition from the liquid to the vapor phase of the fluid iscontemplated a watfle pattern containing multiple conduits may sometimesbe advantageous.

In order better to describe and further clarify the in vention thefollowing is a detail description of embodiments thereof with referenceto the accompanying drawing:

superposed aluminum sheets 1 and 2 having a pattern of a suitable stopweld 3 sandwiched therebetween are illustrated in FIGURE 1. Each of thesheets is degreased by immersing it in an organic solvent such asnaphtha or white gasoline, any oxide clinging thereto is removed byscratch brushing and a pattern of stop weld 3 is applied before thesheets are assembled. Stop weld 3 may conveniently be composed of amixture of about 13% colloidal graphite, about 40% calcium carbonatehaving a granulation of about 325 mesh or less and about 47% water. Thetwo sheets are assembled by lying one on the other in face to facerelationship and are fastened together by spot welding or other means inareas not covered by stop weld 3.

The resulting assembly of sheets 1 and 2 is heated in a suitable furnaceto about 1000 F. in a reducing atmos- 3 phere and while at thistemperature the assembly is passed through a rolling mill to reduce itsthickness about 60%.

Those contacting surfaces of sheets 1 and 2 not protected by stop weld 3are thus forge welded together. The assembly may be rolled later at'room temperature to the final gauge. In some embodiments, of course,the cold rolling step may not be required although it has been foundthat a cold rolling step is advantageous when it is necessary to controlthe final dimensions of the assembly to close tolerances.

The unwelded portions lying within the assembly are inflated by means offluid pressure. As illustrated in FIGURE 2 a hole 4 is drilled in theassembly adjacent the unwelded area and a suitable needle or similardevice 33, in FIGURE 3, is inserted in hole 4 and compressed air isapplied therethrough. A panel such as derived from the assembly ofFIGURES 1, 2 and 3 is illustrated in FIGURE 4. Theconduit resulting fromthe inflation is designated generally as 5 and the surface of the panelis designated generally as 6.

The passageway or hole 4 is also utilized in filling the conduit 5 witha fluid refrigerant. A needle or similar somewhat pointed instrumentillustrated as 7 in FIG- URE 5 is inserted through hole 4 into theconduit and the desired amount of fluid, such as, ethyl alcohol, isallowed to flow in by gravity or it may be pumped in, if desired. Theneedle 7 should be of such dimensions that the air displaced by thefluid in the conduit can escape through hole 4. Conduit 5 is onlypartially filled with a fluid in order to compensate for expansion ofthe fluid When'heated. When the fluid chosen undergoes phase transitionfrom a vapor to a liquid a relatively large condensation area is needed.

After the liquid has been introduced into conduit 5 hole 4 is plugged bysoldering, brazing, cold welding or other suitable processes to completethe fabrication of the circuit through which the fluid will flow. Theresulting panel is then welded, brazed or attached by any other suitablemeans to a tube or other container adapted to contain the heated liquidin the heat exchanger. Such a tube is illustrated as 8 in FIGURE 6 andhas a fin such as is formed from the panel of the preceding figures.Conduit 5 in this fin is of greater dimensions in those areas which willcontain the heated fluid than in the other areas of the circuit. Thedirection of flow of fluid in conduit 5 is illustrated with arrows inFIGURE 6.

It is also possible to form the heat exchanger tube and fins by a forgewelding process similar to that described in the foregoing. In such aprocess two aluminum sheets 11 and 12 in FIGURE 7 are cleaned asdescribed in conjunction With the foregoing embodiment and two patternsof stop weld 13, similar to pattern 3 of the foregoing embodiment, areapplied to the surface of one sheet. A band of stop weld 14 is appliedto one surface of one of the sheets bisecting the sheet and extendingalmost throughout its length such as illustrated in FIGURE 7. Thisassembly is heated, forge welded by rolling to reduce its thickness atleast 65 percent and otherwise processed in accordance with the processdescribed above and the conduits are expanded by drilling a hole in theresulting assembly to provide an entrance to the unwelded areasbordering the patterns of stop weld 13. As indicated in the drawing theband of stop weld 14 terminates short of the edges of the assemblyexcept that a narrower band 15 at one end thereof extends to the edge ofthe sheet. This unwelded portion bordering stop weld 15 is pried apart,a suitable needle is inserted and the unwelded area bordering 14 isthereafter inflated by means of fluid pressure. The assembly is thentrimmed along its edges to remove the passageway resulting from 15 sothat the passageway resulting from 14 extends entirely through toprovide a tube 19 with fins 20 and 21 similar to those as illustrated inFIGURE 8. The integral fins 20 and 21 have conduits 22 and 23 and arefilled with fluid refrigerant substantially as described in conjunctionwith 4 the foregoing embodiments. A cross-sectional view of a fin andtube arrangement of the type illustrated in FIG- URE 8 is shown inFIGURE 9.

A variation in the type of design that can be made in accordance withthis invention is illustrated in FIGURE 10. In this particularembodiment three clean sheets of aluminum are utilized and patterns ofstop weld are sandwiched between each pair of contacting surfaces of theresulting assembly of three sheets. The assembly is forge welded by hotrolling to reduce the thickness of the assembly about 65 percent and theunwelded areas protected by the stop weld are inflated by means of fluidpressure to produce a central tube 19 having fins 20-and 21. Each of thefins has two conduits, one on each side thereof. The center sheet of theassembly of three sheets forms a partitioning member between the pair ofconduits lying on each side of the fin. In the embodiment shown inFIGURE 10 the upper conduit of fin 20 is designated as 24 while theupper conduit of fin 21 is 25. The lower conduit of fin 20 isillustrated as 26 and the lower conduit of fin 21 is designated 27. Itis to be noted that the fins are bent toform an acuate angle with theaxis of the tube in order that gravity can be utilized for return of thecooled liquid from the outer edge of the fin to that portion of theconduit adjacent tube 19. Tube 19 in this instance is in a horizontalposition.

The embodiment illustrated in FIGURE 11 is a circular fin extendingaround the tube 28 which contains the heated liquid. This fin has aplurality of conduits having a portion 31 adjacent the heated tube 28, aportion 29 adjacent the cooler edge 32 and a radially extending portion30 connectingportions 31 and 29. Edge 32 of the fin is at a higherelevation than the edge adjacent tube 28. In operation, the liquidconfined in the conduit is heated to the boiling point and moves towardsportion 29. It condenses there and flows back through the radiallyextending portion 30 to portion 31. The dimensions of the portions 29and 31 can, of course, be

varied but the greater the size thereof the greater the heat transfer insome instances. In some embodiments the portion 29 may be of no greaterlength than the width of the portion 30.

In the foregoing embodiments the fins are formed from aluminum. For somepurposes it might be moredesirable to form the fins from copper orcopper-base alloys or ferrous alloys. Indeed the invention contemplatesforming the fins from any suitable metal. Although it is advantageous toform the fins from sheet metal =by forge welding as described in theforegoingit is, of course, possible to use other welding methods forforming the conduits .on the surface of the fins. For example, theraised portion might be stamped from a sheet of metal and thereafterbrazed or otherwise fastened to the surface of the fin to form aconduit. From the practical standpoint, however, the forge Weldingmethod with subsequent inflation by fluid pressure is favored. Theassembly illustrated in FIGURE 8 is particularly advantageous becausethe fins are integral with the tube and thus the heat transfercharacteristics are improved over a structure where it is necessary toweld or otherwise connect the fin to the surface of the tube. Aplurality of assemblies of this type can be arranged to .form heatexchangers having a tube following a serpentine path with fins extendingtherefrom. The heat exchanger of this invention is particularly usefulfor making automobile radiators or other heat exchangers Where it isdesirable to transfer the heat from a liquid to air.

In one application the fins of this invention can be attached to acylinder suitable for use in internal combustion engines and it ispossible to eliminate the radiator usually required for cooling suchengines. Because of the improved heat transfer characteristics of thefins it is possible to air cool the cylinders. In forming an assembly ofcylinders and fins a mold can be formed by stacking aluminum fins one onthe other with a suitable washer interposed between each pair of fins.This assembly is clamped together and a cylindrical cast iron membersuitable to serve as a cylinder wall is placed inside the resultingcavity. Molten aluminum is then poured between the external wall of thiscast iron cylinder and the wall formed by the alternate aluminum finedges and washer edges. Upon solidification, the edges of the fins arewelded to the aluminum cylindrical Wall and the cast iron cylinderserves as a liner for the cylinder.

In another application of this invention similar to the foregoing thefins are made of rust resistant sheet iron and the spacing Washers areof the same material. The cast iron cylinder is formed by pouring moltencast iron in a suitable mold, the outer wall of which is constituted bythe fins and spacers to which the molten cast iron firmly attachesitself.

By choosing the proper fluid and gas pressure in the conduit thecylinder temperature can be held at the most efficient temperature. Inautomobiles this temperature may be above 212 F. By sealing nitrogen inthe circuit at 60 lbs. gage pressure with water, boiling of the waterwill not occur until about 290 P. so temperature of the auto cylinderand fin would tend to stabilize at this temperature. Very pure watercontaining corrosion inhibitors may be used. Also chemicals such asethylene glycol and the like may be added to prevent freezing.

As indicated hereinbefore it is desirable that that portion of theconduit which is to contain the captive fluid after cooling should be atan elevation greater than that portion which contains the heated captivefluid. In this Way the cooler fluid will more readily flow from the edgeof the fin to that portion adjacent the tube containing the heatedliquid. Circulation within the conduit is also brought about by boilingof the fluid in the conduit. For this reason the fluid contained in theconduit should preferably have a boiling point less than the boilingpoint of the liquid in the heat exchanger tube. For example, if water isto be cooled the conduit may advantageously contain ethyl alcohol or amixture of this and water.

As an illustration of an advantage of this invention, aluminum has aspecific heat of 0.2 calories per gram per degree centigrade temperaturerise while copper has a specific heat of 0.1. If it is desired to keepthe boiling point of a liquid in the heat exchanger tube, such as thatillustrated as 19 in FIGURE 8, at a temperature between that of boilingwater (100 C.) and the boiling point of ethyl alcohol (about 78 C.),ethyl alcohol may be used in the conduit and each gram of alcohol boiledin the portion of the conduit adjacent tube 19 will carry away to theextremities of the fin as much heat as would be required by raising 1020grams of aluminum 1 C. in temperature. If copper is utilized for the fininstead of aluminum 2040 grams of copper would be required for a similarincrease of 1 C. Thus the heat exchanger fin having the enclosedcircuitous conduit containing a fluid permits the use of a fin with muchless weight to preserve the same minimum of temperature differentialbetween heated and cooled portions of the fin.

The configuration of the conduit may be such that a turbulent gas flowbetween adjacent fins is introduced Without impairing the principalfunction of the cooling liquid. In assembling several fins together toform a heat exchanger the inflated portions forming the conduit would bealternated in position in order to cause a turbulence of the air as itpasses between the fins. It is apparent from the foregoing that the finof this invention can be assembled to tubes with a minimum number ofbrazed joints which is, of course, advantageous not only from thestandpoint of manufacturing costs but also with respect to the life ofthe heat exchangers. Although ethyl alcohol or its mixtures with waterare particularly well suited in the fins for cooling heated water anyother suitable fluid such as methyl alcohol glycerine, ethylene glycol,water or its mixtures with the foregoing, xylene, or the like may beutilized. The stop weld composition given in the foregoing isparticularly well suited for aluminum but other compositions such asmixtures of colloidal graphite and sodium silicate can be utilized. Theamount of reduction in the forge welding process will vary dependingupon the particular metal utilized but will ordinarily fall within therange of from 35% to about 65%.

Although embodiments of the invention have been described inconsiderable detail in the foregoing it is to be understood thatmodifications can be made therein by those skilled in the art withoutdeparting from the spirit or scope of the invention except by the scopeof the appended claims.

What is claimed is:

1. A method for making heat exchangers which comprises cleaning theto-be-superimposed surfaces of a plurality of metal sheets, placing aband of stop weld on the said cleaned surface of one of the sheets in apattern adjacent the four edges thereof defining a circuitous system oftubular passageways circumscribing said heat exchanger, said patternbeing coextensive with and spaced from the said edges, forming anassembly by superimposing the cleaned surface of another of said sheetsadjacent said stop-weld coated surface, heating the assembly and forgewelding by hot rolling the adjacent surfaces of said assembly togetherin the areas not separated by said stop-weld material, forming an inletinto the unwelded areas defined by said stop-weld material, inflatingthe unwelded areas by means of fluid pressure injected through saidopening, injecting a fluid in the resulting passageway, sealing the holethrough which the injection was made to seal said fluid in said systemof passageways, and thereafter attaching the resulting panel having aconduit containing the fluid to a heat exchanger tube.

2. A method for making a heat exchanger which cornprises cleaning twometal sheets, applying two separate narrow circuitous patterns of stopweld on the surface of the sheets and a separate narrow band of stopWeld along one axis of said sheet which bisects the same and liesbetween the aforesaid pair of patterns, said stop-weld material beingapplied so that each of said patterns defines a circuitous system oftubular passageways extending through said heat exchanger in a closedcircuit circumscribing a portion of said heat exchanger, welding theface to face portions of the sheet unprotected by the stop weld,inflating the unwelded areas, injecting a heat exchange fluid in thecircuitous conduits resulting from the inflation of the unwelded areasadjacent the aforesaid circuitous patterns of stop weld, and sealingsaid fluid in said circuitous conduits.

3. A method of making a plate-like secondary heat exchanger in heatexchange relationship with a primary heat exchanger comprising formingsaid secondary heat exchanger by sandwiching between pressure weldablesurfaces of superposed flat sheets a pattern of stop-weld materialdefining a circuitous system of fluid passageways extending in a closedcircuit circumscribing at least one portion of said secondary heatexchanger, pressure welding said sheets together in their adjacent areasnot separated by said stop-Weld material, forming an inlet into theunwelded portions of said sheets in the areas defined by said stop-weldmaterial, inflating said unwelded area by means of fluid pressureapplied through said opening to form said system of passageways,injecting a heat exchange fluid through said opening into said system ofpassageways, sealing said opening to seal said heat exchange fluid insaid system of passageways, and placing said secondary heat exchanger inheat exchange relationship with said primary heat exchanger.

4. The method of claim 3 wherein said fluid partially fills saidpassageways and is vaporizable and condensable.

5. The method of claim 3 wherein said pattern is applied to define saidsystem of passageways to comprise at least three tubular passagewayportions extending in at least three directions serially connected toeach other at their ends to form said closed conduit circuit.

-6. The method of claim wherein said fluid partially fills saidpassageways and is vaporizable and condensable.

7. The method of claim 3 wherein said system of passageways is appliedadjacent the edges of said sheets to define a system of passagewayscomprising a continuous tubular portion coextensive With and spaced fromsaid edges.

8. A method of making a plate-like secondary heat exchanger in heatexchange relationship witha primary heat exchanger comprisingsandwiching between pressure weldable surfaces of superposed flat metalsheets a first pattern and separate independent second pattern ofstopweld material, said first pattern being applied to define desiredtubular passageways for flow of a primary heat exchange fluidtherethrough and said second pattern being applied to define acircuitous system of tubular passages extending between said sheets in aclosed circuit circumscribing at least one portion of said sheets,pressure welding said sheets together in the areas not separated by saidstop-weld material, forming an opening into the unwelded areas betweensaid sheet defined by said second pattern of stop-weld material,applying through said opening a fluid pressure of sufiicient magnitudeto expand said unwelded area defined by said second pattern of stop-weldmaterial, injecting through said opening a secondary heat exchange fluidto at least partially fill said unwelded area, and sealing said openingto seal said secondary heat exchange fluid in said unwelded area.

9. The method of claim 8 wherein said secondary heat exchange fluidpartially fills said unwelded area and is vaporizable and condensable.

10. A method of making a plate-like heat exchanger comprising formingsaid heat exchanger by sandwiching between pressure weldable surfaces ofsuperimposed flat sheets a pattern of stop-weld material defining asystem of fluid passages adapted to form a closed system of saidpassages containing a fluid under pressure, pressure welding said sheetstogether in their adjacent areas not separated by said stop-weldmaterial, forming an inlet into the unwelded portions of said sheets inthe areas defined by said stop-weld material, inflating said unweldedareas by means of fluid pressure applied under sufficient pressurethrough said opening to form said system of passageways, injecting aheat exchange fluid through said opening into said system ofpassageways, sealing said opening to form a closed system of saidpassageways containing said heat exchange fluid.

11. A method of making a plate-like secondary heat exchanger in heatexchange relationship with a primary heat exchanger comprising formingsaid secondary heat exchanger by sandwiching between pressure weldablesurfaces of superimposed flat sheets a pattern of stop-weld materialdefining a system of fluid passageways adapted to form a closed systemof said passages containing a heat exchange fluid with at least one ofsaid sheets adapted to be placed in heat exchange relationship with saidprimary heat exchanger, pressure welding said sheets together in theiradjacent areas not separated by said stop-weld material, forming aninlet into the unwelded portions of said sheets in the areas defined bysaid stop-weld material, inflating said unwelded areas by means of fluidpressure applied under suflicient pressure through said opening to formsaid system of passageways, injecting a heat exchange fluid through saidopening into said system of passageways, sealing said opening to form aclosed system of said passageways containing such heat exchange fluid.

12. A method of making a plate-like second heat exchanger in heatexchange relationship with a first heat 'exchanger comprisingsandwiching between pressure weldable surfaces of superimposed flatmetal sheets a first pattern and a separate independent second patternof stop-weld material, said first pattern being applied to definedesired tubular passageways for flow of a first heat exchange fluidtherethrough and said second pattern being applied to define a secondsystem of tubular passages with said second system of tubular passagesadapted to be formed into a closed system containing a second heatexchange fluid, pressure welding said sheets together in the areas notseparated by said stop-weld material, forming an opening into theunwelded areas between said sheets defined by said second pattern ofstop-weld material, applying to said opening a fluid pressure ofsuflicient magnitude to expand said unwelded area defined by said secondpattern of stop-weld material, injecting through said opening saidsecond heat exchange fluid to at least partially fill the expandedunwelded area defined by said second pattern, and sealing said openingto seal said sec- 0nd heat exchange fluid in said expanded unwelded areadefined by said second pattern of stop-weld material.

13. The method of claim 12 wherein said second heat exchange fluidpartially fills said expanded unwelded area defined by said secondpattern of stop-weld material and wherein said second heat exchangefluid is vaporizable and condensable.

References Cited in the file of this patent UNITED STATES PATENTS2,167,901 Murray Aug. 1, 1939 2,690,002 Grenell Sept. 28, 1954 2,754,572Johnson July 17, 19.56 2,756,487 Heidorn July 31, 1956 2,772,180 NeelNov. 27, 1956 2,863,303 Wurtz Dec. 9, 1958

1. A METHOD FOR MAKING HEAT EXCHANGERS WHICH COMPRISES CLEANING TOTO-BE-SUPERIMPOSED SURFACES OF A PLURALITY OF METAL SHEETS, PLACING ABAND OF STOP WELD ON THE SAID CLEANED SURFACE OF ONE OF THE SHEETS IN APATTERN ADJACENT THE FOUR EDGES THEREOF DEFINING A CIRCUITIOUS SYSTEM OFTUBULAR PASSAGEWAYS CIRCUMSCRIBING SAID HEAT EXCHANGER, SAID PATTERNBEING COEXTENSIVE WITH AND SPACED FROM THE SAID EDGES, FORMING ANASSEMBLY BY SUPERIMPOSING THE CLEANED SURFACE OF ANOTHER OF SAID SHEETSADJACENT SAID STOP-WELD COATED SURFACE, HEATING THE ASSEMBLY AND FOREGEWELDING BY HOT ROLLING THE ADJACENT SURFACES OF SAID ASSEMBLY TOGETHERIN THE AREAS NOT SEPARATED BY SAID STOP-WELD MATERIAL, FORMING AN INLETINTO THE UNWELDED AREAS DEFINED BY SAID STIP-WELD MATERIAL, INFLATINGTHE UNWELDED AREAS BY MEANS OF FLUID PRESSURE INJECTED THROUGH SAIDOPENING, INJECTING A FLUID IN THE RESULTING PASSAGEWAY, SEALING THE HOLETHROUGH WHICH THE INJECTION WAS MADE TO SEAL SAID FLUID IN SAID SYSTEMOF PASSAGEWAYS, AND THEREAFTER ATTACHING THE RESULTING PANEL HAVING ACONDUIT CONTAINING THE FLUID TO A HEAT EXCHANGER TUBE.